The present invention relates to the field of seismic exploration sensors. More particularly, the invention relates to a system for selectively locking sensor gimbals to reduce undesirable acoustic noise during seismic operations.
Seismic exploration operations use acoustic sensors to detect energy reflected from subsurface geologic formations. Acoustic energy sources generate energy for penetrating the subsurface geologic formations, and a portion of such energy is reflected upwardly from formation interfaces. Sensors for detecting the reflected acoustic energy, such a velocity detectors, are preferably aligned in a vertical orientation to eliminate signal variables caused by differing sensor orientations.
Conventional sensors use gimbal systems to align velocity detectors with the local vertical. Dual component sensors, three component land velocity sensors, four component sensors, and other seismic equipment incorporate such gimbal systems. The gimbal components may comprise a single gimbal or a primary gimbal with one or more additional gimbals. Gimbal movement during seismic data acquisition introduces noise which interferes with the detected seismic signal. Such noise is caused by contact between the internal components providing gimbal movement, and by correlative movement of the sensors.
Gimbal locks have been used to prevent transport damage to sensitive gimbal components. For example, U.S. Pat. No. 3,554,466 to Paine (1971) disclosed a gimbal lock mechanism for protecting the components during launch of the gimbal payload into orbit. U.S. Pat. No. 5,579,071 to Wetzel et al. (1996) disclosed a self-centering camera lock mechanism which avoided camera distortion as the camera was locked, and U.S. Pat. No. 3,580,363 to Plawner et al. (1971) disclosed a lock for the elevation gimbal of a large telescope. Other systems stablize gimbal movement as shown in U.S. Pat. No. 5,655,412 to Luik (1997).
Seismic data gathering operations produce numerous, consecutive seismic events triggered by the discharge of acoustic energy and the subsequent detection of the reflected signal. Sensor movement during the seismic event, such as in geophones towed behind a moving seismic vessel, inherently introduces variables into the data collection systems. In offshore seismic operations, such movement can occur due to movement of a seismic vessel, wind, waves, and ocean currents. To dampen this motion in geophone housings, viscous fluid can be placed within the interior of the gimbal structure. The viscous fluid limits movement of the geophone sensors and gimbal components by dampening such movement and by preventing extraneous movements.
Viscous damping liquids can negatively impact operation of the sensors. For example, temperature changes significantly change the fluid viscosity in a gimbal structure. A single fluid viscosity provides different damping constants when multiple gimbal masses are used. If the damping fluid is too viscous, the gimbal structure may not have sufficient righting force to respond to orientation and location changes. However, the viscosity of the fluid must be sufficiently great to retard sensor movement during the duration of a single seismic event. Otherwise, the accuracy of data detected and recorded during such seismic event will be affected by the sensor movement during each seismic event.
The stiffness of the damping fluid and the differential, righting mass of the gimbal structure define a time constant. To record a seismic signal with fidelity, the time constant should be significantly longer than the seismic record length. Accordingly, viscous damping fluids inherently require a compromise between motion fidelity between the sensor housing and sensing elements during each seismic event, and the response of the gimbal in returning to a vertical orientation after movement.
There is, accordingly, a need for an improved system for accommodating free gimbal movement of seismic exploration sensors. The system should overcome the problems of conventional fluid dampened gimbal systems and should stabilize the sensor during the pendency of each seismic event.